In recent years, the media industry has expanded its horizons beyond traditional analog technologies. Audio, photographs, and even feature films are now being recorded or converted into digital formats. To encourage compatibility between products, standard formats have been developed in many of the media categories.
As would be expected, the viewers of digital video desire the same functionality from the providers of digital video as they now enjoy while watching analog video tapes on video cassette recorders. For example, viewers want to be able to make the video jump ahead, jump back, fast forward, fast rewind, slow forward, slow rewind and freeze frame.
Various approaches have been developed to provide non-sequential playback of digital video data. With respect to digital video data, non-sequential playback refers to any playback operation that does not play all of the encoded frames in the exact order in the sequence in which they were encoded. For example, jump ahead and fast forward operations are non-sequential in that some frames are skipped. Rewind operations at any speed are non-sequential in that during a rewind operation, frames are not played in the sequence in which they are encoded.
One approach to providing non-sequential playback of digital video data, referred to herein as the tag-based approach, is described in U.S. Pat. No. 5,659,539, entitled "Method and Apparatus for Frame Accurate Access of Digital Audio-visual Information" issued to Porter et al on Aug. 19, 1997, the contents of which are incorporated herein by this reference. According to the tag-based approach, a stored digital video file is parsed to generate "tag information" about individual frames within the file.
Specifically, the tag file contains information about the state of one or more state machines that are used to decode the digital representation. The state information varies depending on the specific technique used to encode the audio-visual work. For MPEG-2 files, for example, the tag file includes information about the state of the program elementary stream state machine, the video state machine, and the transport layer state machine.
During the performance of the audio-visual work, data from the digital representation is sent from a video pump to a decoder. The information in the tag file is used to perform seek, fast forward, fast rewind, slow forward and slow rewind operations during the performance of the audio-visual work. Seek operations are performed by causing the video pump to stop transmitting data from the current position in the digital representation, and to start transmitting data from a new position in the digital representation. The information in the tag file is inspected to determine the new position from which to start transmitting data. To ensure that the data stream transmitted by the video pump maintains compliance with the applicable video format, prefix data that includes appropriate header information is transmitted by the video pump prior to transmitting data from the new position.
Fast forward, fast rewind, slow forward and slow rewind operations are performed by selecting video frames based on the information contained in the tag file and the desired presentation rate, and generating a data stream containing data that represents the selected video frames. The selection process takes into account a variety of factors, including the data transfer rate of the channel on which the data is to be sent, the frame type of the frames, a minimum padding rate, and the possibility of a buffer overflow on the decoder. Prefix and suffix data are inserted into the transmitted data stream before and after the data for each frame in order to maintain compliance with the data stream format expected by the decoder.
The tag-based approach works well when there is enough time between the creation of the original digital video stream and the viewing of the digital video stream to allow the original digital video stream to be parsed to generate tag information. However, when the digital video stream is being viewed as it is being generated, parsing the digital video stream becomes impractical. The amount of computational power required to parse the digital video stream as it arrives would be prohibitively expensive. On the other hand, it is not considered acceptable to increase the latency between the occurrence of many types of video feeds (e.g. sporting events) and the time at which such feeds are available for audience viewing.
When a video stream is made available for viewing before generation of the stream has been completed, the video stream is said to be a "live feed". At a professional level, non-linear digital editors can be used to rapidly review footage of a live feed for a single user. However, these systems are not intended for and cannot be easily adapted to serve many users. For example, if a hundred users were watching the same live feed but wanted to rewind, pause, and fast forward the feed at different times, each would require a separate non-linear digital editor.
Another problem associated with providing non-linear access to live digital video streams is that users may attempt to fast forward into portions of the video stream that do not yet exist. For example, a viewer may attempt to fast forward a live feed to see the final score of game which, in reality, has not yet ended. It is desirable to provide techniques for handling these types of situations in a way that ensures that the decoder will not freeze nor the video stream become corrupted.
Based on the foregoing, it is clearly desirable to provide a method and apparatus for sequentially displaying non-sequential frames of a live digital video. It is further desirable to provide such non-sequential access to live digital video in a way that does not require each viewer to operate prohibitively expensive hardware. It is also desirable to provide safeguards against attempts to access portions of a live digital video stream that do not yet exist.